1. Field of the Invention
The present invention relates to a quasi-soft decision decoder for convolutional self-orthogonal codes, and, more particularly, to a decoder including a demodulator which is augmented to render a quality bit in conjunction with each detected data bit. The quality bit provides an additional vote in the decision of whether a data bit is erroneous or not.
2. Description of the Prior Art
Two systematic procedures are known for threshold decoding of a redundant sequence which includes information signals encoded in accordance with a convolutional, self-orthogonal code. One of these procedures, defined as direct decoding, is embodied in the error-correcting system disclosed in U.S. Pat. No. 3,227,999 issued to D. W. Hagelbarger on Jan. 4, 1966. There, each decoding correction made with respect to an information signal is not fed back to change the nature of a stored syndrome or error pattern representation. If such a change were made, the decoding of subsequently processed information signals would be directly influenced. In other words, in direct decoding there is no feedback connection in the decoding circuitry. Hence, an erroneous decision by the decoder cannot lead to other faulty decisions in subsequent processing. In effect, a positive immunity against error propagation in the decoding process is thereby achieved, but at the expense of reduced error-correcting capabilities.
The other available procedure for processing convolutional self-orthogonal codes is known as feedback decoding. As disclosed in U.S. Pat. No. 3,439,334 issued to J. L. Massey on Apr. 15, 1969, each decoding correction made not only corrects the information bit but is also fed back to the syndrome register to correct, rightly or wrongly, the affected bits of the syndrome register. It is apparent that in a feedback decoder, a bad decoding decision can introduce additional errors in the decoding process.
In conclusion, the prior art decoders for convolutional self-orthogonal codes are usually one of two types: direct decoders or feedback decoders providing hard and soft decision capabilities, respectively.
The problem remaining in the prior art is to provide a decoder which provides reduction in error clustering and better Bit Error Rate (BER) than the hard decision decoding technique, and with reduced cost and complexity from that found in conventional soft decision decoders.